It has long been known in the prior art to provide apparatus for purposes of effecting the grinding and pulverizing of certain materials. More specifically, the prior art is replete with examples of various types of apparatus that have been used to effect such grinding of a multiplicity of materials. Coal is one such material wherein there is a need that it be ground to a particular fineness in order to render it suitable for the use in, for example, a coal-fired steam generating power plan.
One particular coal pulverizing apparatus, which is to be found in the prior art, is a roller mill. An exemplary roller mill with an integral classifier is shown and described in U.S. Pat. Nos. 4,640,464 and 7,028,847. An exemplary roller mill 10 having an integral classifier 12 is depicted in FIG. 1. As is typical of such a vertical roller mill, the mill includes a plurality of grinding rolls 14 that toll along a grinding ring 16 for pulverizing the material to a desired particle size, which defines a grinding zone 18. A blower (not shown) generates an upward airflow 20 that draws fine particles upward through a grinding chamber 22 to the classifier 12 disposed above the mill housing 24 and in fluid communication therewith. The classifier has a centrifugal-type separator 25 that separates the oversize particles from the finer particles. An airflow 28 having finer particles pass through the classifier 12 through an output duct or port 26 while the airflow 30 having oversized particles fall back down to the grinding chamber 22 and rolls 14 for continued grinding. The airflow 30 also includes fine particles. Some of these fine particles may continue to circulate within the grinding chamber 22 without being further ground as shown at 34.
It has been found that when the roller mill in the prior art is used for grinding fine particle sizes, the roll tends to pound the ring and generate high vibration and noise levels. It is believed that this high vibration may be due to one or more factors. One factor is the lack of sufficient material in between the rolls and ring. The ground material (e.g., particles) is so fine that the particles are very easily blown away by the upward airflow. Another factor is the skidding or sliding of the roll along the grinding ring because the fine particles disposed between the roll and ring are so fine that the fine particles act like a lubricant between the roll and ring.
What is needed, therefore, is a means for overcoming or at least reducing the severity of the increased vibration, noise level, and/or roll skidding as described hereinbefore. The improved fine grinding roller mill roll and grinding ring design disclosed herewith reduces these problems by keeping more material in the grinding area and results in an increase mill throughput or decrease of the mill power consumption.